The present invention relates in general to waveguide filters, and more particularly to an improved and relatively simple design of a waveguide low pass filter.
There presently exists various types of waveguide low pass filters. Although these structures are usually adequate for their intended use, many times they are relatively complex to fabricate and therefore expensive. There is a need for improvement to reduce the size and cost of these filters and yet improve their performance.
The use of corrugated ridge waveguides is well known. For example, one such structure is shown in Very High Frequency Techniques, Vol. II, McGraw Hill, 1947, pg. 736. This type of a filter is known to have a multitude of undesirable spurious responses. If it is desired to use the corrugated ridge waveguide in association with a standard waveguide, once again there are spurious responses that occur especially when higher order modes are excited. With these structures the gaps between the broad walls must generally be made quite small to reduce the spurious responses. This produces a low impedance structure which is difficult to match to a standard rectangular waveguide. A further reference to this structure is found in Microwave Filters, Impedance Matching Networks and Coupling Structures, by Matthaei et al., McGraw-Hill, 1964, Sec. 7.0.4.
The standard solution to spurious modes has been to provide axial slots in the structure which produces the well known "waffle filter". This type of a structure does not however improve the low impedance problem and does add to the machining cost for the structure.
Accordingly, one object of the present invention is to provide an improved waveguide low pass filter that can be fabricated relatively inexpensively and yet provide for a minimizing of spurious responses.
Another object of the present invention is to provide a low pass filter structure that can operate at moderately high average power with a very low insertion loss.
Still another object of the present invention is to provide a waveguide low pass filter that may be constructed smaller than present comparable filters such as a waffle iron filter.